The major goal is to understand the structural basis of immune recognition and the detailed chemistry and diversity of antibody- antigen interactions. Two of the most important antibody recognition events which currently require understanding concern antipeptide antibodies and catalytic antibodies. The common theme of these projects is antibodies raised against synthetic antigens, designed as immunological tools to answer important structural questions. The first objective is to determine and refine three- dimensional structures by x-ray crystallography at high resolution (less than 1.9-2.1 A) of an anti-peptide Fab, peptide-Fab complexes and protein-Fab complex in order to understand how an antibody can recognize the same antigenic determinant as a free peptide and as part of an intact viral antigen. The structural recognition of different peptide sequences and specifically designed conformationally-restricted analogues will be correlated with affinity measurements in order to quantitate antibody-antigen interactions. The structures of the free peptide determined by N.M.R. (Dyson et al., 1985), the peptide bound to the Fab and the peptide in the intact influenza virus hemagglutinin (Wilson et al., 1981) will be compared to evaluate antigen conformational changes which occur on antibody binding as well as any changes which occur in the antibody. The second objective is to determine the three dimensional structure by x-ray crystallography of a catalytic Fab and designed transition-state analogue-Fab complexes, and hence, how monoclonal antibodies can act as enzymatic catalysts (abzymes). The catalytic antibody to be studied initially has esterolytic activity and was raised against phosphonate haptens specifically constructed and designed to act as transition state analogues when bound to the antibody. In addition, the structural study will investigate the differential rate enhancement of different esterolytic antibodies. Such results will contribute not only to providing new insights into enzyme catalysis but show how the diversity of the immune system can be manipulated to provide potentially powerful biological, medical and biotechnological tools of exquisite stereochemical specificity. In both these systems crystals of both the free Fab and Fab-antigen complex are available and hence one can evaluate for the first time whether any changes in antibody structure accompany antigen binding.